The present invention pertains to a clamping circuit for restricting the voltage of the input signal to a prescribed range, and interface circuit that makes use of said clamping circuit.
In the prior art, in order to increase the ESD (Electrostatic Discharge) voltage rating and to suppress the reflected noise for the interior of IC, wiring of PCB (Printed Circuit Board), etc., a diode clamping circuit or Schottky barrier diode clamping circuit is used. FIG. 7 is a diagram illustrating an example of the Schottky barrier diode clamping circuit.
As shown in FIG. 7, this clamping circuit is composed of Schottky barrier diodes (referred to as diodes hereinafter) D1 and D2 connected in series between the supply line of power source voltage Vcc and ground voltage GND.
The anode of diode D1 is connected to input terminal Tin, and its cathode is connected to the supply line of power source voltage Vcc. The anode of diode D2 is grounded, and its cathode is connected to input terminal Tin.
In this case, if the forward rise voltage of diode D1 is VD1, and signal voltage Vin of input terminal Tin satisfies the condition Vin greater than Vcc+VD1, diode D1 becomes conductive, voltage Vin of input terminal Tin is clamped to a level above power source voltage Vcc by the conductive voltage of diode D1.
If the forward rise voltage of diode D2 is VD2, and signal voltage Vin of input terminal Tin satisfies the condition Vin less than VD2, diode D2 becomes conductive, and voltage Vin of input terminal Tin is clamped to a level below ground voltage GND by the conductive voltage of diode D1.
FIG. 8 is a diagram illustrating the clamping characteristics of the clamping circuit shown in FIG. 7. As shown in the figure, this clamping circuit may be connected to the terminal of a signal transmission line to clamp the terminal voltage of the signal transmission line from a voltage slightly below ground voltage GND to a voltage slightly above power source voltage Vcc. In this way, it is possible to prevent reflection of the signal at the terminal of the transmission line and to inhibit generation of reflected noise on the transmission line.
However, in the aforementioned conventional clamping circuit, the range of the clamping voltage depends on power source voltage Vcc and the conductive voltage of the diode used in the clamping circuit, and it is impossible to set the clamping voltage at will. Also, due to the conductive voltage of the diode, the clamping voltage range shifts a little from power source voltage Vcc and ground voltage GND. For example, when voltage Vin at input terminal Tin is to be clamped between ground voltage GND and power source voltage Vcc, voltages Vc1 and Vc2 should be supplied to the cathode of diode D1 and the anode of diode D2,respectively, as shown in FIG. 9. Consequently, voltage sources for generating said voltages Vc1 and Vc2 must be arranged separately. This is undesirable for the terminal circuit which should have a simple circuit configuration.
The purpose of the present invention is to solve the aforementioned problems of the conventional technology by providing a clamping circuit which has a simple circuit configuration and which can set the clamping voltage range at will, and an interface circuit that makes use of said clamping circuit.
In order to realize the aforementioned purpose, the present invention provides a type of clamping circuit characterized by the following facts: the clamping circuit has a first circuit containing a first transistor and a first diode electrically connected in series between the input terminal and the fit voltage feed line, and a first voltage feed terminal for feeding the first voltage to the control terminal of said first transistor; when the voltage between said first voltage feed terminal and said input terminal reaches a prescribed level, said first transistor becomes conductive and the voltage of said input terminal is clamped.
Also, the clamping circuit of the present invention is characterized by the following facts: the clamping circuit has a first circuit containing a first transistor and a first diode electrically connected in series between said input terminal and the first voltage feed line, a first voltage feed terminal for feeding the first voltage to the control terminal of said first transistor, a second circuit containing a second transistor and a second diode electrically connected in series between said input terminal and the second voltage feed line, and a second voltage feed terminal for feeding the second voltage to the control terminal of said second transistor; when the voltage between said first voltage feed terminal and said input terminal or the voltage between said second voltage feed terminal and said input terminal reaches a prescribed level, said first or second transistor becomes conductive and the voltage of said input terminal is clamped.
Also, the clamping circuit in the present invention is preferably characterized by the following facts: it has a first, a second and a third resistive element connected in series between said first voltage feed line and said second voltage feed line; the connection point between said first resistive element and said second resistive element forms said first voltage feed terminal; and the connection point between said second resistive element and said third resistive element forms said second voltage feed terminal.
Also, the clamping circuit of the present invention is preferably characterized by the following facts: the cathode of said first diode is connected to said input terminal; said first transistor is connected between the anode of said first diode and said first voltage feed line; the anode of said second diode is connected to said input terminal; said second transistor is connected between the cathode of said second diode and said second voltage feed line; the power source voltage is fed to said first voltage feed line; and the ground voltage is fed to said second voltage feed line.
The interface circuit of the present invention is characterized by the following facts; the interface circuit has a signal line, a drive circuit which has its output terminal connected to said signal line and is for driving said signal line to the prescribed voltage, an input circuit which has its input terminal connected to said signal line and operates corresponding to the voltage of said signal line, and a clamping circuit connected to the input terminal of said input circuit.